In the casting of metals in plate molds it is often a requirement that the resulting castings have an even thickness. In many cases, however, variations in thickness occur as a result of the method of casting.
When molten metal is cast in horizontal plate molds into sheets, plates or slabs variations in thickness of the casting occur because of wave action in the molten metal. The wave action is a result of the pouring of molten metal into the mold and the freezing of the molten metal. If a crest of a wave impinges on the mold wall at the moment that the metal is ready to solidify, the metal freezes to the thickness of the wave crest, whereas at another time it may be a valley in the wave that freezes. This wave phenomenon, therefore, causes the thickness of the plate casting to vary.
Uneven thickness of plate castings is also caused by the formation of a flash of metal at the edge of the casting. Waves that are created in the main body of the casting hit the side wall and climb up onto it. Because the side wall is usually at a lower temperature the skin of the wave crest in contact with the mold freezes, while the remaining of the wave drops back. This results in thin projections (flash) along the edges of the casting.
Uneven thickness of a plate casting is a particular problem with electrodes that are used in metal electro-deposition processes.
Electrodes having integral lifting lugs for metal electro-deposition processes are traditionally cast in apparatus that includes a number of molds placed at the perimeter of a casting wheel. Variations in thickness in the lifting lugs of the electrodes, caused by the wave phenomenon, make it unsafe and difficult to pick up a number of electrodes by one lifter having a number of grabs that have to fit between the electrodes and then twist to hook under the lugs. Because of thickness variations, an operator can not manipulate the grabs over the electrodes to ensure that every electrode in the group is securely held in the grabs before lifting. A tolerance has to be designed into the grabs which will mean that invariably certain electrodes will fall off the grabs or prevent the grabs from moving under the lugs.
The occurrence of a flash of metal at the edges of the electrodes is a source of electrical shorts during electro-deposition that must be accommodated in the cells by providing sufficient, i.e. increased, spacing between electrodes.
Although book molds can be used for making copper bullion electrodes for use in a copper refinery, book molds are not suitable for casting electrodes of lead bullion for use in the electrorefining of lead by the Betts or the bipolar process. The use of bookmolds for lead bullion causes variations in crystal structure in the electrodes that are not inducive into retaining slime on the electrode, and slime falls into the electrolyte to the detriment of the electrolytic process.
Book molds are also unsuitable for casting the thin anodes used in the electrowinning of zinc. The anodes are usually made of a lead-silver-calcium alloys. The presence of air during casting in a book mold causes oxidation and entrapment. The entrapped air causes porosity in which the oxygen is consumed by the calcium forming a vacuum. When the oxygen is consumed while the electrode is residing in the cell, the vacuum will draw electrolyte into the cavity through micro cracks in the metal. The electrolyte then attacks the oxides along the grain boundaries resulting in very rapid and severe corrosion.